Spectroscopy of electronic transitions in Polycyclic Aromatic Hydrocarbon cations and their clusters

This thesis is an experimental study of the electronic spectroscopy of cations of Polycyclic Aromatic Hydrocarbons (PAHs) and their aggregates in conditions close to those of the interstellar medium (ISM), i.e. cold and totally isolated in the gas phase. It is related to the astrophysical context of the interstellar medium (ISM), in particular on the question of the possible link between interstellar PAHs and Diffuse Interstellar Bands (DIBs). The purpose of this thesis is to provide laboratory spectra which can be directly compared to the spectra of DIBs. Indeed these bands are the oldest spectroscopy riddle in astrophysics which remained unanswered for nearly 100 years and whose key is still looked for. A special attention is given to the methylated derivatives of PAHs species detected in many interstellar environments, cationic PAH dimers (the simplest PAH clusters). These clusters have been proposed as a model of the very small grains, which contribute to the formation of interstellar PAHs and whose chemical composition remains uncertain. This thesis has been mainly devoted to the determination of the electronic spectra of naphthalene cation monomer (Np⁺) and its methylated derivative (2-MeN_p⁺), as well as the associated homogeneous dimers. The experimental method used is based on the photodissociation of van der Waals complexes PAH⁺_m-Ar_n (argon atoms spectators), prepared by UV laser photoionization in a supersonic jet. This technique combines different experimental tools, namely: molecular beam mass spectrometry and laser spectroscopy as well as physical tools such as the handling of clusters VdW PAH⁺_m-Ar_n, the detection of photo-fragments, the measurement of photodissociation efficiency. The identification of the fragments by the photodissociation of VdW clusters allowed us to determine the different possible fragmentation channels and especially to obtain the spectra the charge resonance transition and the first allowed transition to locally excited states of the homogeneous dimer cations cited above. In complement to these experimental achievements, a theoretical study has been made in the context of the main astrophysical signatures of PAHs, the series of mid-infrared emission bands known as the Aromatic Infrared Bands for AIBs. These spectra are discussed and interpreted in the light of earlier works. This study consisted in calculations of structures and infrared spectra of neutral and cationic dehydrogenated naphthalene derivatives using the Density Functional method. The vibrational data derived from DFT calculations were then used by the welcoming team as input for numerical simulations, using a kinetic Monte Carlo procedure, which allowed the determination of the infrared emission spectrum expected from the heating of the naphthalene molecule by 6.4 eV. (author)